1. Field of the Invention
This invention relates to the interpolation of scan lines to convert the resolution of an image, and in particular to the interpolation of scan lines during the conversion of a video signal from interlaced scanning to progressive scanning.
2. Description of the Related Art
In the interlaced scanning system, one frame of a video signal is divided into two fields. When the frame is displayed, first one field is scanned; then the other field is scanned. The scan lines of the two fields occupy alternate positions on the display screen, so that a typical scan line of the first field is positioned adjacently between two scan lines of the second field, and a typical scan line of the second field is positioned adjacently between two scan lines of the first field. In the progressive scanning system, a frame comprises only one field, and each scan line in the field is adjacent to other scan lines in the same field.
The conversion of a video signal from interlaced scanning to progressive scanning requires the separate generation of two video signal frames from the first and second fields of an interlaced video signal. Scanning conversion is therefore performed by interpolating scan lines between the scan lines of each field of the interlaced video signal.
If the interlaced video signal is of a nonmoving image, so-called inter-field interpolation is generally performed: the scan lines in one field are inserted between the scan lines of the other field, so that the scan lines of both fields are used in each frame. If the interlaced video signal is of a moving image, so-called intra-field interpolation is performed: each frame is generated from the scan lines of just one field, the interpolated scan lines being generated by averaging the values of picture elements (pixels) on the adjacent scan lines. Examples are shown in FIG. 4 of Japanese Unexamined Patent Application Publication (hereinafter, JP) H3-179890 and FIG. 1 of JP 2002-1122003.
An image may also be divided into a nonmoving part and a moving part, and inter-field interpolation and intra-field interpolation may be performed to interpolate scan lines in the nonmoving part and moving part, respectively, as disclosed in JP H3-179890.
One scanning conversion system, disclosed in JP H3-179890, interpolates scan lines by generating pixels through vertical averaging of the pixel values on adjacent scan lines. If the image being converted includes diagonal lines or edges oriented at an angle to the scan line direction, however, pixel interpolation by vertical pixel averaging alone produces blurred or jagged boundaries.
Another scanning conversion system, disclosed in JP 2002-1122003, eliminates blurred or jagged boundary lines by selecting the most strongly correlated pair of pixels from among the pairs of pixels disposed at point-symmetrical positions on opposite sides of the pixel to be interpolated. Such a pair of pixels will be referred hereinafter to as a ‘pixel pair’.
The conventional scanning conversion systems described above both have difficulties with images including fine lines oriented at small angles to the scan line direction. That is, it is difficult to generate the pixels for these fine lines correctly on an interpolated scan line.
In the scanning conversion system disclosed in JP H3-179890, in which interpolated scan lines are generated by averaging the video signals (pixel values) of vertically adjacent scan lines, an interpolated pixel on an interpolated scan line is given the average value of the pixel immediately above it and the pixel immediately below it. If the interpolated pixel lies on a very fine line, however, and the pixels immediately above and below the interpolated pixel do not form part of the very fine line, then the fine line will disappear at this point. After scanning conversion, fine lines oriented at small angles to the scan lines tend to be displayed as discontinuous lines with a dotted or dashed appearance.
In the scanning conversion system disclosed in JP 2002-1122003, in which correlations between pixel pairs are obtained, the level of correlation is determined from differences between the value of a pixel block including a predetermined number of pixels surrounding one pixel in the pixel pair and the value of a pixel block including the same number of pixels surrounding the other pixel in the pixel pair. The value of a pixel block (the pixel block value) is calculated as the sum of the pixel values in the block, or as a weighted sum of these pixel values. With this method, however, the direction of very fine diagonal lines may be unidentifiable because of identical differences between pixel block values in two or more pixel pairs, in which case the best pixel pair to use for generating the interpolated pixel cannot be selected unambiguously. After scanning conversion, the fine line may therefore be displayed as a discontinuous line.